Vince, in the shop manual for the 1MZFE (at least in my 2002 but its probably the same of rthe 1999) the trouble areas are listed in an order for each code.
Under code P0171 it lists in this order:
Air intake (hose loose)
Fuel line pressure
Injector blackage
open or short in A/F sensor circuit
A/F sensor malfunction
Intake flow meter
Rather than test the two items before the AF sensors and then test the AF sensor and then test the MAF sensor as the inspection procedure illustrates, it sounds like the dealer wanted to just "test by replacement" and hope it goes away. I'm guessing on this next one but I think the MAF sensor replacement is overall less expensive so they go with the mose profitable.?.
steviej
←
Sorry, I have spoke to soon. The problem has come back. Both the CEL and the Track off light have illuminated and the engine has begun to run a little rough. I have also noticed that the catalyic converter is getting real hot, reason being that it is pinging when I shut down the car. I have the print outs from the diaganostic machine from lexus, but cannot make out what it really means? It has percentages next to each cylinder 1-6 with the error codes. But Inskip Lexus insisted that it was the O2 sensor? Question#1 Can all the experts here on the forum reevaluate this problem? Thanks Much Vince
←
Vince, Missfire is caused by compression loss, coils, wires, plugs, cam timing... 20 years, never seen an O2 sensor cause missfire. What are the % next to each cyl? I think you need to pull all 6 plugs and wires and check plugs for hairline carbontracks....looks like pencil lead down the porcilin. Spray some of your leftover brake clean down each plug bore to remove any moisture traces. Ohm check plugwires when they are out...8-15Kohms is nominal. Does your printout give a reading next to cal load? what about MAF g/sec? injector M/sec? post these if it is on there along with short term fuel trim. We can get this. I think you fixed 1 problem but are still looking at missfire caused by water intrusion that has carbon tracked a plug or two. Look/listen for vac leaks hoses off at right side of the plenum.